In recent years, near-infrared absorbing dyes that have absorption in the near infrared range and thus are useful for information recording materials utilizing laser light (for example, optical cards, organic optical electroconductive bodies, laser thermal transfer recording, laser thermosensitive recording, laser direct plate making and the like), apparatuses for which near-infrared absorbability (or heat ray absorbability) is required (for example, near-infrared absorbing filters, filters for plasma displays, optical filters, optical filters for thin displays, optical filters for optical semiconductor elements, heat ray shielding films, heat ray shielding resin glasses, protective eyeglasses, secret inks, agricultural films, glasses, interior or exterior materials for automobiles, resin molded articles and the like), and the like have been increasingly demanded.
As near-infrared absorbing dyes, cyanine-based dyes, polymethine-based dyes, squarylium-based dyes, porphyrin-based dyes, metal dithiol complex-based dyes, phthalocyanine-based dyes, diimonium-based dyes, inorganic oxide particles and the like are used.
However, the near-infrared absorbabilities of these compounds are not satisfiable, and these compounds have absorption wavelengths in the visible ray range, and thus it was impossible to use them for uses for which transparency is required and applications for which coloring is not preferable. Specifically, in the cases when these near-infrared absorbing dyes are used as near-infrared absorbers, they are used by combining with synthetic resins such as thermoplastic resins in many cases, and in such cases, problems occurred in compatibility with the resins, the transparency of the resins was damaged due to the absorption wavelengths that the dyes have in the visible ray range, problems of coloring occurred, or other physical properties of the resins were damaged in many cases.
Therefore, a compound that has a maximum absorption wavelength in the near infrared range and has low absorption in the visible ray range has been demanded.
Furthermore, use of (heavy) metal elements is sometimes problematic, for the problem of environmental burden, or depending on an intended use such as the field of precise electronic materials, and thus a near-infrared absorber other than metal complexes is desired.
On the other hand, Non-patent Literature 1 describes an N indigo complex, but this is a β-diketoiminate metal (palladium) complex, and thus the finding of the present invention cannot be obtained from this complex.